1. Field of the Invention
The invention relates to a process and an apparatus for preparing vinyl chloride monomer (hereinafter “VCM”) and pertains to operating a column for separating off hydrogen chloride from the mixture obtained in the thermal dissociation of 1,2-dichloroethane (hereinafter “DCE”). Specifically, the invention pertains to heating the HCl column in the VCM plant or in an integrated plant for preparing both DCE and VCM.
The invention is directed to a process for preparing VCM which is usually obtained by thermal decomposition of DCE and from which polyvinyl chloride (hereinafter “PVC”) is ultimately produced. The conversion of DCE into VCM forms hydrogen chloride (hereinafter “HCl”). DCE is therefore preferably prepared from ethylene and chlorine in such a way that a balance in respect of the hydrogen chloride produced and consumed in the reactions, as per the following reaction equations, is achieved:Cl2+C2H4—C2H4Cl2 (pure EDC)+180 kJ/Mol   (1)C2H4Cl2 (dissociation EDC)−C2H3Cl (VCM)+HCl−71 kJ/Mol   (2)C2H4+2 HCl+½O2—C2H4Cl2 (crude EDC)+H2O+238 kJ/Mol   (3)
2. Description of the Related Art
In processes for preparing vinyl chloride by incomplete dissociation of DCE, the DCE used is usually vaporized in the first step, the vapor formed is then pyrolytically dissociated at relatively high temperature in a second step, the entrained solids are then, in a third step, separated from the hot dissociation gas produced in the second step and the purified dissociation gas is subsequently fed to work-up by distillation.
HCl and VCM are formed as main products in the dissociation of DCE carried out in the second process step.
As by-products, traces of soot, chlorinated and unsaturated hydrocarbons and also benzene are obtained.
To limit the formation of these undesirable by-products, the temperature in the dissociation is kept at a level which leads to incomplete conversion of the DCE. The hot dissociation gas produced by dissociation in the second process step therefore still contains unreacted DCE in addition to the main products HCl and VCM and the abovementioned by-products.
The dissociation of EDC to form VCM is an endothermic process, and occurs in the gas phase in the form of pyrolysis. The pyrolysis is carried out industrially in the absence of a catalyst under a high pressure of from 1 to 3 MPa and at a temperature of from 450 to 600° C. However, catalytic processes which allow the pyrolysis to be carried out at lower temperature are also known.
The hot dissociation gas produced by means of pyrolysis is obtained at the pyrolysis temperature. It is conditioned so that it assumes a form suitable for the actual separation of materials. For this purpose, it is quenched in a quenching column, in which solids present in the dissociation gas are also scrubbed out. The solids are taken off at the bottom of the quenching column. The major part of the gaseous mixture of DCE, VCM and HCl (referred to as quenching vapor or quenching top vapor) is taken off at the top of the quenching column and passed to further work-up. Before the further work-up, the heat content of the quenching vapor can be utilized economically in one or more heat exchangers.
A plant complex for the production of vinyl chloride (hereinafter referred to as “VCM complex”) consists essentially of:                a plant for preparing DCE from ethene and chlorine (“direct chlorination”, optional plant component); and        a plant for preparing DCE from ethene, hydrogen chloride and oxygen (“oxychlorination”); and        a plant for purifying 1,2-dichloroethane by distillation (preparation of “feed DCE”); and        a plant for the thermal dissociation of the “feed DCE” which has been purified by distillation into vinyl chloride and hydrogen chloride; and        a plant for separating off the hydrogen chloride and unreacted 1,2-dichloroethane by distillation and also for purifying the vinyl chloride.        
The hydrogen chloride obtained by thermal dissociation of 1,2-dichloroethane is recirculated to the oxychlorination plant and once again reacted there with ethene and oxygen to form DCE.
Numerous measures for saving energy or recovering heat in plants for the preparation of DCE, VCM and PVC are known from the prior art. Such measures lead to a significant decrease in the operating costs and thus contribute quite substantially to the economics of the plant. Likewise, such measures also contribute significantly to reducing the CO2 emissions from the plant.
These include measures which utilize the heat of reaction of the exothermic reaction steps in order to heat heat sinks in the process. Thus, for example, the heat of reaction from the oxychlorination is used to produce steam by means of which, for example, feed preheaters or distillation columns can be heated.
There have likewise been proposals for utilizing heat energy originating from the VCM plant. An example of such processes may be found in DE 34 40 685 A1. Here, the vapor from the high-boiler column is mechanically compressed and used for heating the same column. The DCE obtained in the circulating convection vaporizer of the high-boiler column by condensation of the mechanically compressed vapor is then used as feed DCE in the thermal dissociation after it has been preheated further by the flue gases from the dissociation oven. In another process variant described in WO 2004/089860 A1, the vapor from the quenching column downstream of the dissociation oven is utilized for preheating the feed DCE.
DE 31 47 310 A1 discloses a method of recovering heat in the preparation of VCM by dissociation of DCE. Here, heat is recovered from dissociation gases produced in the preparation of VCM and used for heating distillation columns. The dissociation of DCE is carried out under superatmospheric pressure and the dissociation gases are quenched by direct cooling. In an example, the thermal energy obtained is used for operating distillation columns downstream of the quenching column.
DE 29 13 004 A describes a method of recovering pyrolysis energy in the preparation of vinyl chloride by incomplete thermal dissociation of 1,2-dichloroethane. In this document, it is proposed that steam be generated in a heat exchanger by means of the heat content of the hot dissociation gases from the dissociation oven and this steam then be used for heating columns.
DE 35 19 161 A1 discloses a process for purifying 1,2-dichloroethane. Here, a distillation column is operated under superatmospheric pressure and the heat content of the overhead stream is used for heating further heat sinks. The dichloroethane obtained is so hot that is used for heating 1,2-dichloroethane-comprising product streams.
Recently, plant concepts in which the DCE column is operated in the mode of “pressure distillation” have been proposed. A consequence of this is that the feed DCE (for the thermal dissociation to form VCM) obtained in this process has a considerably increased temperature compared to earlier processes. This removes the necessity of preheating this feed DCE before the dissociation, or preheating requires considerably less heat energy than in the case of earlier processes. Hitherto, the feed DCE was heated mainly by the heat content of the dissociation quenching vapor. This heat source could now be passed to other uses or the dissociation quenching vapor would have to be cooled in another way, for example in an air-cooled condenser.